1. Field of the Invention
The present invention relates to a method of manufacturing fuel cells by fixing a polymer electrolyte film to a frame. The present invention also pertains to fuel cells manufactured according to this method.
2. Description of the Related Art
The fuel cells convert the chemical energy of a fuel directly to electrical energy. In a general configuration of the fuel cell, a pair of electrodes are arranged across an electrolyte layer. A supply of gaseous fuel containing hydrogen is fed to one electrode, that is, the anode, whereas a supply of oxidizing gas containing oxygen is fed to the other electrode, that is, the cathode. The fuel cells of this structure generate an electromotive force through the electrochemical reactions proceeding on the anode and the cathode. One major problem of the fuel cells is that the mixture of the gaseous fuel and the oxidizing gas fed to the respective electrodes lowers the efficiency of power generation. In the fuel cells, it is accordingly required to prevent the flow of gaseous fuel from being mixed with the flow of oxidizing gas.
Polymer electrolyte fuel cells have a polymer electrolyte film as the electrolyte layer. The polymer electrolyte fuel cells are generally manufactured by laying a large number of unit cells one upon another to form a stuck structure. In each unit cell, a polymer electrolyte film (hereinafter may be simply referred to as electrolyte film) is interposed between a pair of gas diffusion electrodes to form a sandwich-like structure. A pair of gas-impermeable separators are further disposed across the sandwich-like structure. The electrolyte layer separates the flow of gaseous fuel from the flow of oxidizing gas. The separator prevents the flow of gaseous fuel and the flow of oxidizing gas from being mixed with each other in the adjoining unit cells. In the polymer electrolyte fuel cell, in order to prevent the flow of gaseous fuel from being mixed with the flow of oxidizing gas, the electrolyte film is bonded to the separators with an adhesive in each unit cell to ensure the gas sealing property of the electrodes.
Even when an appropriate adhesive is selected by taking into account the materials of the electrolyte film and the separator, the fuel cell manufactured according to the prior art technique may have the insufficient adhesive strength between the polymer electrolyte film and the separator. The insufficient adhesive strength may result in damaging the gas sealing property of the electrodes. This causes the fuel cell manufactured to have the poor reliability for the gas sealing property of the electrodes.
The object of the present invention is thus to enhance the adhesive strength of a polymer electrolyte film via an adhesive and thereby manufacture a fuel cell having the high reliability for the gas sealing property.
At least part of the above and the other related objects is realized by a first method of manufacturing a fuel cell by fixing a polymer electrolyte film to a frame. The first method includes the steps of: causing the polymer electrolyte film to have a water content of not greater than 4, which is expressed as a molar fraction of H2O; and bonding the polymer electrolyte film to the frame with an adhesive.
In the first method of the present invention, the polymer electrolyte film treated to have the water content (expressed as the molar fraction of H2O) of not greater than 4 is bonded to the frame with the adhesive. As is known, the water content of the polymer electrolyte film varies with a variation in humidity of the atmosphere. The water content of the polymer electrolyte film may abruptly increase according to the relative humidity. In the case where the polymer electrolyte film has a large water content during manufacture of the fuel cell, a large quantity of water molecules are adsorbed by the functional groups in the polymer electrolyte film. This undesirably lowers the adhesive strength of the polymer electrolyte film via the adhesive. The first method of the present invention causes the polymer electrolyte film to have the water content (expressed as the molar fraction of H2O) of not greater than 4. This effectively prevents a large quantity of water molecules from being adsorbed by the functional groups and thereby ensures the sufficient adhesive strength of the polymer electrolyte film via the adhesive.
The first method of the present invention does not lower the adhesive strength of the polymer electrolyte film via the adhesive even when the atmosphere has a high humidity during manufacture. The fuel cell manufactured according to the first method of the present invention has the high reliability for the gas sealing property between the polymer electrolyte film and the frame.
The present invention also provides a second method of manufacturing a fuel cell by fixing a polymer electrolyte film to a frame. The second method includes the steps of: providing an adhesive having a modulus of elasticity of not greater than 10 MPa after cure; and bonding the polymer electrolyte film to the frame with the adhesive.
In the second method of the present invention, the polymer electrolyte film is bonded to the frame with the adhesive having the modulus of elasticity of not greater than 10 MPa after cure. This arrangement enables the adhesive layer between the polymer electrolyte film and the frame to be readily expanded and contracted in the fuel cell thus manufactured. In this fuel cell, even when the polymer electrolyte film is expanded or contracted with a variation in humidity of the atmosphere after the cure of the adhesive, the adhesive layer can follow the expansion or the contraction. This effectively prevents the polymer electrolyte film from being hardened or broken and protects the adhesive layer from the damage.
The fuel cell manufactured according to the second method of the present invention has the high reliability for the gas sealing property between the polymer electrolyte film and the frame even in the service environment of a remarkable humidity change.
The present invention further provides a third method of manufacturing a fuel cell by fixing a polymer electrolyte film to a frame. The third method includes the steps of: providing an adhesive having a durometer A hardness of not greater than 90 after cure; and bonding the polymer electrolyte film to the frame with the adhesive.
In the third method of the present invention, the polymer electrolyte film is bonded to the frame with the adhesive having the durometer A hardness of not greater than 90 after cure. This arrangement enables the adhesive layer between the polymer electrolyte film and the frame to be in a relatively soft state in the fuel cell thus manufactured. In this fuel cell, even when the polymer electrolyte film is expanded or contracted with a variation in humidity of the atmosphere, the adhesive layer can follow the expansion or the contraction. This effectively prevents the polymer electrolyte film from being hardened or broken and protects the adhesive layer from the damage.
Like the second method of the present invention, the fuel cell manufactured according to the third method of the present invention has the high reliability for the gas sealing property between the polymer electrolyte film and the frame even in the service environment of a remarkable humidity change.
In the first method of the present invention, it is preferable that the step of bonding the polymer electrolyte film comprises placing the adhesive having a modulus of elasticity of not greater than 10 MPa after cure and/or a durometer A hardness of not greater than 90 after cure.
Even in the case where the atmosphere has a high humidity during manufacture or in the case where there is a large variation in humidity in the service environment, this configuration ensures the high reliability for the gas sealing property between the polymer electrolyte film and the frame of the fuel cell.
In the second method of the present invention, it is preferable that the step of providing the adhesive comprises providing the adhesive having a durometer A hardness of not greater than 90 after cure. Even in the case where there is a large variation in humidity in the service environment, this configuration ensures the high reliability for the gas sealing property between the polymer electrolyte film and the frame of the fuel cell.
In any of the first through the third methods of the present invention, in accordance with one preferable structure, the step of bonding the polymer electrolyte film comprises providing the frame being a pair of separators that are arranged across a pair of gas diffusion electrodes, between which the polymer electrolyte film is interposed.
In the fuel cell manufactured according to the method of this preferable structure, the polymer electrolyte film separates a flow of gaseous fuel, which is fed to one gas diffusion electrode arranged on one side of the polymer electrolyte film, from a flow of oxidizing gas, which is fed to the other gas diffusion electrode arranged on the other side of the polymer electrolyte film. The separators seal the flows of gaseous fuel and oxidizing gas. This fuel cell ensures the high reliability for the gas sealing property between the polymer electrolyte film and the separators. This arrangement ensures the gas sealing property of the gas diffusion electrodes without using any special elements, such as O-rings.
In any of the first through the third methods of the present invention, the adhesive may be a modified rubber adhesive containing a mixture of epoxy resin and modified silicone. The adhesive may also include resin beads of a predetermined diameter.
The present invention is directed to a first fuel cell, which includes: a frame; and a polymer electrolyte film that has a water content of not greater than 4, which is expressed as a molar fraction of H2O, and is bonded to the frame with an adhesive.
The first fuel cell corresponds to a fuel cell manufactured according to the first method of the present invention discussed above. Like the first method, the first fuel cell ensures the high reliability for the gas sealing property between the polymer electrolyte film and the frame.
The present invention is also directed to a second fuel cell, which includes: a polymer electrolyte film; a frame; and an adhesive that is used to bond the polymer electrolyte film to the frame and has a modulus of elasticity of not greater than 10 MPa after cure.
The second fuel cell corresponds to a fuel cell manufactured according to the second method of the present invention discussed above. Like the second method, the second fuel cell ensures the high reliability for the gas sealing property between the polymer electrolyte film and the frame.
The present invention is further directed to a third fuel cell, which includes: a polymer electrolyte film; a frame; and an adhesive that is used to bond the polymer electrolyte film to the frame and has a durometer A hardness of not greater than 90 after cure.
The third fuel cell corresponds to a fuel cell manufactured according to the third method of the present invention discussed above. Like the third method, the third fuel cell ensures the high reliability for the gas sealing property between the polymer electrolyte film and the frame.
These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiment with the accompanying drawings.